Malaria study challenges warmer world predictions

A world with “rampant” malaria transmission is often seen as an inevitable consequence of global warming. But a new study radically challenges existing ideas of how the disease will spread with rising temperatures.

Previous models have predicted that the optimal temperature for transmission is 31 °C. The new model suggests this is 25 °C. It also suggests that transmission would drastically decrease above 28 °C.

“Past models showed the whole world was going to light up with malaria – it was quite terrifying,” says Kevin Lafferty, the senior investigator on the study, who is based jointly at the University of California, Santa Barbara, and the US Geological Survey. But the new model, which takes into account the effects of temperature on a variety of key aspects of insect and parasite physiology, suggests a different pattern.

It agrees with past models that cooler areas without malaria – such as the African highlands, parts of Europe and the US – may warm up to give conditions ripe for transmission. But places currently optimal for malaria transmission may not remain so in a warmer world, says Erin Mordecai, also at the University of California, Santa Barbara, who led the study.

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Cold-blooded

The new model focuses on a key factor used to calculate the risk of spread of a disease called R0, or the Basic Reproduction Number. This measures the number of secondary cases that arise from a single case in a susceptible population.

Past models have assumed a relatively simple relationship between R0 and temperature. The Anopheles mosquito which carries the parasite is cold-blooded, so models have typically assumed that as temperature increases, so does the propensity for the spread of malaria.

But it is known that aspects of the life history of the Anopheles mosquito and the malaria-causing parasite it carries increase with temperature to a certain point and then rapidly decline above that temperature.

The new model combines data from other studies on how factors like mosquito bite, parasite development and mosquito egg-laying rates change with temperature. Crucially, the team found their model’s predictions matched real data from 14 countries in Africa on the rate at which people were bitten by infectious mosquitoes.

The model has received mixed reactions from other experts in the field. “It’s a welcome advance from what has been done before, making better and more realistic associations about climate and malaria links,” says Richard Ostfeld, a disease ecologist at the Cary Institute of Ecosystem Studies in Millbrook, New York. But he adds that “great caution” needs to be taken in using this new optimal transmission temperature in terms of predicting climate effects on malaria.

Other factors

In fact, the study authors themselves highlight some of the problems. For example, on one measure they had to use some data from a different genus of mosquito other than the Anopheles, and on another parameter they used data from the Plasmodium vivax parasite rather than P. falciparum, which causes the malaria more common in Africa.